One approach to non-contactless sensing of torque in a shaft rotating about its axis is a torque sensor based on magnetoelasticity. A magnetoelastic transducer element is secured to or integral with the shaft, the torque in which is to be measured, and a torque-dependent magnetic field emanated by the transducer element is detected by a sensing device external to the shaft, but not in contact with the shaft, and responsive to the emanated magnetic field. Examples of sensing devices are a Hall effect device, a saturating coil sensor, or various of other magnetic field sensitive devices known in the art. It will be understood that in practice a sensing device may be an assembly of devices. For example, a plurality of sensing devices may be disposed about the axis of the shaft and interconnected to be additive with respect to the torque-dependent field but to cancel in respect of external fields such as the Earth's magnetic field.
Magnetoelastic transducer elements form a ring or annulus which is circumferentially magnetised. The field forms a closed loop normally contained within the element.
One form of transducer element is a separate ring of magnetoelastic material attached to the shaft such as disclosed in U.S. Pat. Nos. 5,351,555, 5,465,627 and 5,520,059, all to Garshelis and assigned to Magnetoelastic Devices, Inc. In the ring transducer elements, the ring supports a circumferential magnetic field which is confined within the ring, that is no field is detectable externally in the absence of torque. When torque in the shaft is transmitted to the magnetoelastic ring, an external magnetic field is emanated and is detected by a sensor arrangement.
A different approach to providing a circumferentially magnetised magnetoelastic sensor is disclosed in International Patent Application PCT/GB99/00736 (published on 4th Nov., 1999 under the number WO99/56099) in which the transducer element is an integral portion of the shaft whose torque is to be measured. This avoids problems in securing a separate ring properly to the shaft. An integral transducer element approach is also disclosed in published International Patent Applications WO99/21150 and WO99/21151.
Magnetoelasticity is a phenomenon which, as yet, is apparently still not fully understood and explained. It is, therefore, generally desirable to find other forms of magnetisation that might be employed in transducer elements, particularly suitable for torque sensing.
In an alternative to circumferential magnetisation one or more magnetic transducer elements are provided integrally in a shaft of magnetisable material but using longitudinal magnetisation, that is a magnetisation that lies in an axial direction in contrast to circumferential magnetisation. The longitudinal magnetisation is applied to an annular region of a shaft or more than one such region.
Longitudinal magnetisation is the subject of our copending International application PCT/GB00/03119 filed concurrently herewith and published under the number WO01/13081.
Both circumferential magnetisation and longitudinal magnetisation have been primarily applied to shafts or similar parts where torque is transmitted through the magnetised region in the direction of the shaft axis, e.g. a load transmitting shaft having torque applied at one end and a load at the other end.
There is a need to measure torque in parts in which the load transmission is essentially radial. An example is a disc-like structure mounted on a driven shaft and having drive means at its outer periphery, such as gear teeth. The drive may be in the opposite direction.
One proposal for torque measurement in an automobile transmission is disclosed in U.S. Pat. No. 4,697,460 (Sugiyama et al). An energiser coil/detection coil assembly is non-contactingly placed adjacent a disc in which torque stress occurs. The energising coil is A. C. energised to establish an alternating magnetic flux in a flux path through the disc that is torque sensitive. The detection coil senses torque-dependent changes in the circulating magnetic flux.